Hyperfine coupling constants of organic radicals

The isotropic hyperfine coupling constants of several organic radicals including CH3, CH2, CH2−, C2H5, C2H3, H2CN, C6H7, and C3H5 are calculated analytically using the coupled cluster (CC) “relaxed density’’ matrix approach. We employ three different commonly used basis sets with CCSD and CCSD(T) in order to calibrate expected accuracy. The Chipman basis set combined with the CCSD(T) method performs best for carbon isotropic hyperfine coupling constants with a mean absolute deviation within 8% compared to experiment. The corresponding mean absolute deviation for hydrogen isotropic hyperfine coupling constants from experiment is 12%. We show that the UHF, ROHF, and quasi (QRHF) reference function CCSD spin densities are effectively numerically equivalent in the notorious case of the allyl radical.